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An API response that looks old does not prove the origin returned stale data. A browser or shared cache may be reusing a still-fresh response, or it may have revalidated a stored copy and received a 304 Not Modified. To find out what happened, inspect the full HTTP request and response—especially Cache-Control, Age, validators, and the status code—before blaming the API.
How an API can look stale while behaving correctly
HTTP caching can let a client reuse a stored representation instead of fetching a new one for every request. Whether that reuse is allowed depends on the applicable freshness rules, request and response directives, and any revalidation—not simply on how old the data looks in an application screen.
RFC 9111 describes the purpose of the header this way: “The Cache-Control header field is used to list directives for caches in the request/response chain.” The standard defines both freshness behavior and restrictions on when stale responses may be generated, so interpret the complete exchange rather than one timestamp or header in isolation. RFC 9111: HTTP Caching
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Cache-Control and Expires: the freshness policy
Cache-Control directives govern whether a response may be stored, reused, or revalidated by browsers and shared caches. A response’s max-age sets its freshness lifetime in seconds; it does not mean “this many seconds since this particular client received it.” Check the directives in context, including those on the request, rather than treating max-age as a simple timer that starts at the moment you saw the response. Where present, Expires is another header to inspect when assessing freshness. MDN: Cache-Control
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Age and Date: clues about elapsed time
Age indicates how many seconds an object has been in a proxy cache. A nonzero value can help explain why a response appears older than expected, but it is only a clue: it does not prove that a cache caused a bug or reveal the entire path the response took. Compare it with Date and the applicable freshness lifetime, and account for shared caches between the client and origin. MDN: Age
ETag and conditional requests: checking whether a stored copy changed
An ETag identifies a representation and can be used as a validator. A client that has a stored copy may send its validator in If-None-Match, asking whether that representation has changed. Last-Modified and If-Modified-Since provide another validator and conditional-request path to check. MDN: ETag
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What a 304 Not Modified response means
A 304 Not Modified response is not a newly transmitted representation body. It tells the client that its stored representation can be reused because the validator matched. The body the application displays may therefore come from the client’s stored copy even though the server participated in validating it. Inspect the conditional request and its response together; a 304 has a different meaning from a 200 carrying a representation. MDN: 304 Not Modified
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Trace a response before diagnosing the origin
- Capture one complete exchange. Record the full URL, method, relevant request headers, response status, and response headers. Redact credentials and personal data before sharing logs.
- Assess freshness. Read
Cache-Control,Age,Date, andExpireswhen present. Compare the response’s age with its freshness policy, taking shared caches on the network route into account. - Check for revalidation. Compare a prior response’s
ETagorLast-Modifiedwith a later request’sIf-None-MatchorIf-Modified-Since. Determine whether the result was a304that permits reuse or a200with a representation. - Compare clients or paths carefully. Preserve the same URL and relevant request headers when comparing behavior. A changed request context can select a different representation, so a UI timestamp alone is not enough to identify the source of the data.
- Separate HTTP caching from application data. If the observed headers and exchange do not account for the result, investigate application-level caches and the data source separately. HTTP headers alone do not establish what those layers did.
What the available evidence can—and cannot—establish
Without the endpoint, request and response headers, client, intermediaries, and logs, it is not possible to determine whether a particular incident came from a browser cache, a proxy, application-level caching, or the origin itself. The protocol does establish how to evaluate freshness and validator-based reuse; identifying the cause in a specific case requires the actual exchange and, if necessary, evidence from the other layers.
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